This application is related to U.S. patent application entitled "Torque Multiplier for Aircraft Propeller," to Hora et al., filed on Jul. 23, 1990, as Ser. No. 556,676, the disclosure of which is incorporated by reference.
FIG. 1 illustrates an aircraft 3 powered by counter rotating propellers 6 and 9. The propellers rotate in opposite directions as indicated by arrows 12 and 15. FIG. 2 illustrates a type of turbine system which can be used to drive the propellers 6 and 9. In FIG. 2, fore propeller 6 is attached to a first turbine 18 which is supported by bearings 21 and rotates in direction 12 indicated in FIG. 1. The components rotating in this direction are decorated with hatching in FIG. 2. The aft propeller blade 9 is fastened to a second turbine 24 which is supported by bearings 27 and rotates in direction 15 of FIG. 1. A hot, high energy gas stream 30 provided by a gas generator (not shown) causes rotation of the turbines.
Propellers 6 and 9 (which are sometimes called fans or propfans in the art) are of the variable pitch type. Variable pitch means that each blade can rotate about a respective pitch axis 6A or 9A in FIGS. 1 and 2 as indicated by circular arrows 33 and 36. A principal reason for changing pitch is to provide the blades 6 and 9 with the angle of attack which is proper for the present air speed of the aircraft and power level of the engine.
A problem arises with the configuration of FIG. 2 when one wishes to use a motor 38 supported by a stationary frame 37 in order to change pitch of the forward propeller 6. The problem is that frame 40 of the aft turbine 24 acts as a rotating obstacle which the mechanical forces which change pitch of propeller 6 must cross. That is, one cannot run a shaft from motor 38 as indicated by phantom shaft 42 because such a shaft would lock the two turbines 18 and 24 together and prevent relative rotation.
One approach to crossing this obstacle is given in U.S. Pat. No. 4,657,484, by Wakeman et al., entitled "Blade Pitch Varying Means," of which the General Electric Company is the Assignee, and which is hereby incorporated by reference. Part of this patent is shown in schematic, simplified form in FIG. 3. The reader is referred to the patent itself for precise detail.
In FIG. 3, pitch change is caused by two gear racks 84 and 86 which drive two pinions 82 and 68. The racks 84 and 86 slide left- and rightward on rails 58, 74, and 78. Rail 74 rides on carriage 59, which rides on rail 58. Rail 58 is supported by stationary frame 305, and does not move. However, rail 78 is supported by turbine 24, and rotates along with that turbine.
Rack 86 is driven as follows. A hydraulic piston 50 drives a rod 52 which moves a first inner bearing race left- and rightward, and which pulls a first outer bearing race 60 in the same direction, because of balls located between the races. The balls perform two functions: (1) they pull the outer race 60 along with the inner race 56, thereby forcing the outer race 60 to remain adjacent the inner race 56, and (2) the balls also allow the outer race 60 to rotate about axis 67, while the inner race 56 remains stationary.
The outer race 60 moves the rack 86 left- and rightward and this motion rotates the pinion 68, which rotates a shaft 109, which rotates aft propeller blade 9 in order to change pitch, as indicated by arrow 70. A second hydraulic piston 50A drives a second push rod 52A which moves a second inner bearing race 94 left-and and rightward. The second inner bearing race 94 pulls a second outer bearing race 92 in the same direction. The second outer race 92 moves a second rod 93 left-and rightward. The second rod 93 connects to a third inner race 96, the left- and rightward motion of which drives the second rack 84 left- and rightward through a third outer race 98, in order to change pitch of the fore blade 6.
Second outer race 92 and third inner race 96 are supported by turbine 24, as is second rod 93, and the three components rotate about axis 67. However, the third outer race 98 is carried by turbine 18, as is rack 84 and pinion 82, and these three components rotate along with turbine 18, in the opposite direction.
That is, third races 96 and 98 form a differential bearing: both races rotate in opposite directions. Such is not the case with races 92 and 94, nor with races 56 and 60.
Therefore, axial (ie, left- and rightward) motion of the piston rods 52 and 52A cause pitch change of respective blades 6 and 9, even though turbine 18 is nested within turbine 24. (The former turbine 18 is supported on bearings 321 and 324, while the latter turbine 24 is supported on bearings 308 and 311, and both turbines rotate in opposite directions.)
Five significant features of the arrangement of FIG. 3 are the following. One, as described, the pitches of blades 6 and 9 are independently adjustable. However, there is not true independence of pitch: for reasons which need not be understood by the reader, the system of FIG. 3 requires that the pitches of blades 6 and 9 remain within about 8 degrees of each other. Under some flight conditions, such as during reverse thrust applied after touch-down during landing, it can be required that the pitches of blades 6 and 9 differ by 8 degrees or more.
Two, there is a reversing axial load applied to the bearings associated, for example, with races 96 and 98. This reversing load is not desirable.
Three, the train of components located between the pistons 52 and 52A and the respective blades 6 and 9 possess a certain springiness, which is undesirable. That is, the system is not stiff.
Four, for reasons which need not be understood by the reader, the system of FIG. 3 was found to impose an undesirable travel limit on the total pitch excursion from the deepest negative pitch to the highest positive pitch.
Five, high dynamic loads were found to be imposed on the differential bearing comprising races 96 and 98.
Moreover, in some situations, the use of hydraulic components can be disadvantageous, and so a replacement for piston 50 may be sought.